In a constant velocity universal joint, which is used to construct a power transmission system for automobiles and various industrial machines, two shafts on a driving side and a driven side are coupled to each other to allow torque transmission therebetween, and rotational torque is transmitted at a constant velocity even when each of the two shafts forms an operating angle. The constant velocity universal joint is roughly classified into a fixed type constant velocity universal joint that allows only angular displacement, and a plunging type constant velocity universal joint that allows both the angular displacement and axial displacement. In a drive shaft for transmitting power from an engine of an automobile to a driving wheel, for example, the plunging type constant velocity universal joint is used on a differential side (inboard side), and the fixed type constant velocity universal joint is used on a driving wheel side (outboard side).
Irrespective of the plunging type and the fixed type, the constant velocity universal joint includes, as a main component, an outer joint member including a cup section having track grooves formed in an inner circumferential surface thereof to engage with torque transmitting elements, and a shaft section that extends from a bottom portion of the cup section in an axial direction. In many cases, the outer joint member is obtained by integrally forming the cup section and the shaft section by subjecting a rod-like solid material (bar material) to plastic working such as forging and ironing or a process such as cutting, heat treatment, and grinding.
Incidentally, as the outer joint member, an outer joint member including a shaft section that is long (long stem) may sometimes be used. In order to equalize lengths of a right part and a left part of the drive shaft, the long stem is used for an outer joint member on the inboard side that corresponds to one side of the drive shaft. The long stem is rotatably supported by a rolling bearing. Although varied depending on vehicle types, the length of the long stem section is approximately from 300 mm to 400 mm in general. In the outer joint member, the shaft section is long, which causes difficulty in integrally forming the cup section and the shaft section with high accuracy. Therefore, there is known an outer joint member in which the cup section and the shaft section are constructed as separate members, and both the members are joined through friction press-contact. Such a friction press-contact technology is disclosed in, for example, JP 2012-57696 A.
An overview of the friction press-contact technology for the outer joint member disclosed in JP 2012-57696 A is described with reference to FIGS. 14 and 15. An intermediate product 71′ of an outer joint member 71 includes a cup member 72 and a shaft member 73, which are joined through the friction press-contact. As illustrated in FIG. 14, burrs 75 are generated at inner and outer diameters on a joining portion 74 through the press-contact. In order to mount a rolling bearing (see FIG. 1) to a shaft section of the intermediate product 71′ of the outer joint member 71, as illustrated in FIG. 15, it is necessary to remove the burrs 75 on the outer diameter side of the joining portion 74 through a process such as turning. Although not shown, the intermediate product 71′ is processed into a finished product of the outer joint member 71 through machining of a spline, snap ring grooves, and the like. Therefore, the outer joint member 71 and the intermediate product 71′ have slight differences in shape, but illustration of the slight differences in shape is omitted in FIG. 15 to simplify the description, and the outer joint member 71 as the finished product and the intermediate product 71′ are denoted by the same reference symbols at the same parts. The same applies to the description below.